We plan to continue our investigation of the changes in electrophysiological behavior that give rise to abnormal rhythms of the heart and that accompany the termination of abnormal rhythms. Intracellular microelectrodes are used to record membrane potentials and to inject current in small Purkinje fibers, and in small strips of coronary sinus tissue, both isolated from dog hearts. Both types of preparation are studied in a fast perfusion system to allow rapid alteration of the ionic composition of the extracellular solution or rapid addition or removal of drugs. We plan to use our detailed knowledge of the characteristics of electrogenic sodium extrusion in Purkinje fibers to identify the contribution made by pump current to the Purkinje fiber action potential and to analyze the actions of drugs which possibly alter pump activity. We plan also to continue our study of steady-state currents flowing through excitable channels in the membrane of Purkinje cells. We plan to continue our studies of the factors determining the excitability of cells in the coronary sinus, including the effects of epinephrine on the resting potential and on the appearance of after-depolarizations and triggering.